1. Field of the Invention
The present invention relates to a sintered compact for a tool which is prepared by cubic boron nitride (hereinafter referred to as cBN), and particularly to an improvement in a cBN compact suitably used for an end mill.
2. Description of the Prior Art
Cubic boron nitride is the hardest substance next to diamond, and sintered compacts thereof are applied to various cutting tools. Japanese Patent Laying-Open Gazette No. 77811/1978 disclosed an example of such a cBN sintered compact applicable to a cutting tool.
The prior art discloses a hard sintered compact which mainly contains 80 to 40 percent by volume of cubic boron nitride and a residue of carbide, nitride, boride or silicide of a transition metal selected from groups IVa, Va and VIa of the periodic table, a mixture thereof or a mutual solid-solution compound thereof or those further comprising Si and/or Al. Such a compound is continuous in bonding phase in the structure of the sintered compact. This hard sintered compact for a tool employs carbide, nitride, boride or silicide of a transition metal selected from the groups IVa, Va and VIa of the periodic table, a mutual solid-solution compound thereof or the like. Such compounds are relatively hard and of high melting points. Therefore, sintered compacts prepared by these compounds generally present high performance in application to cutting tools.
A harder sintered compact is preferred in case of using the cBN sintered compact as a high hard sintered compact for cutting tools. Therefore, as described above, the compact containing high volume of cBN has been used. However, in case of the compact being applied to an end mill among cutting tools for cutting high hard materials, even the high hard sintered compact described above is frequently broken in an initial stage of cutting.